For more than 30 years now, a scientific field gathering the so-called "disciplinary didactiques" (Caillot, 2007) is growing in the French-speaking countries, accounting for the irreducible role of the content knowledge in teaching and learning processes. In briefly recalling the rationale of this research domain in France, I will first feature the important theoretical developments pioneered by the didactiques of mathematics (Brousseau, 1997; Chevallard 1985/91) as the epistemological roots of a Joint Action Theory in Didactiques (Sensevy & Mercier, 2007). More importantly, I will show that this new approach (growing within a network for comparative research in didactiques) results from the empirical challenge of studying the ordinary teaching and learning practices in the various subject matters taught at school. From a selection of examples captured in mathematics lessons, I will present the main analytical categories of the Joint Action Theory in Didactiques (Sensevy et al., 2005; Ligozat & Leutenegger, 2008) and its recent developments to address the observable transactions in the classroom as a collective reconstruction of the epistemic practices crystallized in curriculum texts (Ligozat, in press).